1. Field of the Invention
The present invention relates to an aluminum alloy forged material suitably used for a chassis member, structural member and the like for an automobile, and a method for manufacturing the same.
2. Description of the Related Art
Conventionally, for structural materials of transportation vehicles such as railway vehicles, marine vessels, aircrafts, motorcycles or automobiles and the like, aluminum alloys such as a 6000 series (Al—Mg—Si-based) and the like stipulated in JIS standards or AA standards (may be hereinafter abbreviatingly expressed as an “Al alloy”) have been used. This 6000 series aluminum alloy is comparatively excellent in corrosion resistance also, and is excellent also in recycling performance allowing scraps thereof to be reused as melting raw material for the 6000 series aluminum alloy.
Also, for the structural materials of vehicles for transportation, from the viewpoints of lowering the manufacturing cost and working into components of a complicated shape, aluminum alloy cast materials and aluminum alloy forged materials have been used. Out of them, for strength members in which the mechanical properties such as high strength, high toughness and the like are required, that is the chassis members for an automobile such as an upper arm, lower arm and the like for example, the aluminum alloy forged materials have been mainly used.
These aluminum alloy forged materials are manufactured by subjecting the aluminum alloy cast materials to homogenizing heat treatment, thereafter to hot forging such as mechanical forging, oil hydraulic forging and the like, and thereafter to refining treatment such as solution heat treatment, quenching treatment, artificial aging treatment (may be hereinafter simply referred to also as aging treatment) and the like. Also, in order to forge an aluminum alloy, extruded materials obtained by subjecting the cast materials to homogenizing heat treatment and thereafter to extrusion working may be used.
In recent years, in the strength members of these transportation vehicles, because of increasing requirements of low fuel consumption and low CO2 emission, requirements of further weight reduction (thinning) have been raised. Although 6000 series aluminum alloy forged materials such as 6061, 6151 and the like have been used for these applications so far, their performances are insufficient in strength and toughness.
In order to solve such problem, as described in JP-A No. 2001-107168, the present inventors proposed before a high strength and high toughness aluminum alloy forged material excellent in corrosion resistance including Mg: 0.6-1.8% (mass %, hereinafter the same), Si: 0.6-1.8%, further including one or two elements of Cr: 0.1-0.2% and Zr: 0.1-0.2%, restricting Cu: 0.25% or less, Mn: 0.05% or less, Fe: 0.30% or less, hydrogen: 0.25 cc/100 g-Al or less respectively, the remainder being Al and unavoidable impurities, in which the average grain size of Mg2Si and Al—Fe—Si—(Mn, Cr, Zr)-based crystallized and precipitated products present on the grain boundary of the aluminum alloy structure was made 1.2 μm or less, and the average interval between these crystallized and precipitated products was made 3.0 μm or more.
However, although it was clarified that the aluminum alloy forged material described in JP-A No. 2001-107168 was excellent in corrosion resistance, the transition elements represented by Mn, Cr, Zr were less, therefore the crystal grains were liable to be coarsened by recrystallization, and variation in tensile strength became extremely large. When application to chassis components of an automobile is assumed particularly, highly reliable tensile strength is required. Accordingly, when the variation in tensile strength was large, the tensile strength used in designing lowered, and development in such use became hard which became a problem.